The present invention relates to the field of polishing, in particular chemical mechanical polishing in the manufacture of semiconductor wafers and integrated circuits. More particularly, the invention relates to a method and apparatus for controlling chemical-mechanical polishing processes by monitoring the results and process of polishing plate wear and conditioning.
Polishing processes play significant role in modern technologies, in particular in semiconductor fabrication. For example, at certain stages in the fabrication of devices on a substrate, it may become necessary to polish or planarize a surface of the substrate before further processing. Polishing may also be performed with chemically active abrasive slurry, such polishing is commonly known as a chemical mechanical polishing (also called chemical mechanical planarization, or just CMP). In a polishing process, a polishing plate repetitively passes over the surface of the substrate; abrasive particles are either present on the plate or supplied with the slurry. In contrast with mechanical polishing, the CMP slurry provides an increased removal rate of a substrate material and the capability to selectively polish certain films on the substrate.
Chemical mechanical planarization may be used as a preparation step in the fabrication of substrates or semiconductor wafers to provide substantially planar front and backsides thereon. CMP is also used to remove high elevation features and other discontinuities, created on the outermost surface of a substrate during fabrication of a microelectronic circuitry.
The planarization method typically requires that the substrate be mounted in a wafer head or carrier, with the substrate surface to be polished exposed. The substrate supported by the head is then placed against a moving polishing pad mounted on a platen. The head may also move, for example rotate, to provide additional motion between the substrate and the polishing pad. Polishing slurry is supplied onto the pad to provide an abrasive chemical solution at the interface between the pad and the substrate. Pressure is applied on the carrier to effectuate polishing. In some polishing machines the substrate rotates while the polishing pad is stationary, in others the pad moves while the wafer is stationary, and in yet another type both the wafer carrier and the pad move simultaneously. The polishing pad may be pre-soaked and continually re-wet with slurry.
U.S. Pat. No. 5,597,341 issued on Jan. 28,1997 to Kodera, et al, U.S. Pat. No. 5,234,867 issued on Aug. 10, 1993 to Schultz, et al., and U.S. Pat. No. 5,232,875 issued on Aug. 3, 1993 to Tuttle, et al illustrate several techniques and corresponding types of CMP systems for chemical mechanical planarization of semiconductor wafer surfaces.
One type of CMP systems, used in the apparatuses of the type disclosed in the aforementioned references, is shown schematically in FIG. 1a. In this system a polishing pad 10a is mounted on a platen 12a, which rotates by means of a first motor 14a through a transmission 16a. A wafer 20a with a front surface 22a to be polished is held on a head 24a. In the illustrated apparatus, the polishing pad 10a has a diameter significantly larger than that of the wafer 20a (FIG. 1a). The polishing head 24a is rotated by means of a second motor 26a through a transmission 28a and comprises a retaining ring 30a which prevents the wafer from slipping out of the head during polishing. A slurry feeding system 32a pours slurry on the top working surface of the pad 10a. 
FIG. 1b illustrates another embodiment of the aforementioned known CMP system. In this embodiment, a polishing pad 11b is mounted on a platen 12b, which is rotated by means of a first motor 14b through a transmission 16b. A wafer 20b with a front surface 22b to be polished is held on a head 24b. In the illustrated apparatus, the polishing pad 1b has a diameter significantly smaller than that of the wafer 20b (FIG. 1b). The polishing head 24b is rotated by means of a second motor 26b through a transmission 28b and comprises a retaining ring 30b, which prevents the wafer from slipping out of the head during polishing. A slurry feeding system 32b pours slurry on the front surface of the wafer 22b. 
In order to provide uniformity of polishing, in the CMP systems of the types shown in FIGS. 1a and 1b, the distance between the polishing pad rotational axis and the wafer rotational axis is typically varied in an oscillatory manner. For this purpose, the substrate is repeatedly moved back and forth relative to the polishing pad. In FIGS. 1a and 1b the oscillatory movement is shown by arrows 25a and 25b, respectively. Another type of the CMP system, shown schematically in FIG. 2, is disclosed, e.g., in U.S. Pat. No. 5,899,800, issued on May 4, 1999 to Shendon and in U.S. Pat. No. 6,184,139, issued on Feb. 6, 2001 to Adams, et al. In the CMP apparatus of these patents, the lower head comprising a polishing pad 10c mounted on a platen 12c is driven into orbital movements by means of an orbital drive 34 with a motor 36, while the carrier 24c holding the wafer 20c rotates about its central axis by a motor 26c via a transmission 28c. The pad diameter is slightly larger than the diameter of the wafer 20c. Polishing slurry is introduced directly through the openings 38a, 38b . . . 38n in the polishing pad 10c with point-of-use mix, which may result in better wafer uniformity and reduced slurry consumption.
Important characteristics of a planarization process in semiconductor wafer fabrication are a wafer material removal rate and uniformity of material removal. There are several factors that may affect these parameters. Since various materials of the wafer, polishing pad, slurry, and retaining ring interact in a course of polishing, a combination of their characteristics and process parameters, such as compression force or pressure, speed, temperature, etc., can provide specific polishing characteristics Typically, the material combination is selected based on a trade off between the polishing rate, which determines in large part the throughput of wafers through the polishing apparatus, and the need to provide a particular desired finish and flatness on the surface of the wafer.
The efficiency of polishing greatly depends on the pad surface conditions and may reduce with time as a polishing pad is contaminated, its pores jammed with various waste, and worn out. Therefore in the course of polishing, the pad surface should be refreshed or xe2x80x9cconditionedxe2x80x9d after a period of use to provide for both a more uniform polishing rate from wafer to wafer and better planarization uniformity across a single wafer. During the pad conditioning process, a pad conditioner with an abrasive surface is forced to come in contact with the working surface of the pad while both the pad and the conditioner move at pre-determined speeds and pressure. While the operation of conditioning is an effective way of deterring the wear of the polishing pad, the pad requires replacement when either its surface conditions are not recovered by conditioning or its thickness drops below a pre-determined level.
The pad conditioning can be done, for example, with a conditioning device described in U.S. Pat. No. 5,486,131 issued in January 1996 to Cesna, et al. The conditioner has an either circular or ring configuration and is provided with a combination of vertical motion, rotation around a vertical axis, and oscillating horizontal movement.
To ensure good pad flatness during the above conditioning process, the U.S. Pat. No. 5,868,605 issued in February 1999 to Cesna, suggests supplying both pad and conditioner with oscillating radial motions, with a stroke of reciprocation sufficient to have the conditioner extending over the edges of the polishing pad.
Another way of conditioning with optimum pad shape is suggested in the U.S. Pat. No. 5,941,761 issued in August 1999 to Nagahara, et al. A rigid end effector is attached to the above-mentioned conditioner with a concave region to produce a dome-like pad shape.
The above three conditioning methods do not monitor the conditioning process, which dramatically reduces their accuracy. A process operator does not receive any information on when to stop conditioning in order to avoid excessive pad wear, when to replace the conditioner or the pad, and how to adjust the polishing process depending on the state of the pad, thus risking to lose the quality of polishing.
Pad conditioning can be performed both in-situ, that is, during and simultaneously with polishing, and ex-situ, that is, before the polishing. In either case, there is a need for continuous monitoring the pad surface condition in order to make the following decisions:
axe2x80x94how to adjust the conditioning process to achieve the best pad performance,
bxe2x80x94when to accelerate (if there is insufficient conditioning) or decelerate (if there is excessive pad wearing) the in-situ conditioning,
cxe2x80x94when to stop polishing for ex-situ conditioning,
dxe2x80x94when the pad is worn out to such extent that additional conditioning would be useless and the pad has to be replaced,
exe2x80x94when the conditioner is either worn or contaminated, and so requires either replacement or cleaning, correspondingly.
An example of conditioning control is described in U.S. Pat. No. 5,951,370 issued in September 1999 to Cesna. Pad thickness is monitored with laser elements at the inner and outer pad diameters, and compared. If it fluctuates substantially, a control signal is produced, which causes an appropriate adjustment movement of the conditioner to ensure the target flatness of the pad. This method controls pad flatness, but does not monitor any other crucial characteristics of pad wear and surface condition.
Another example of the conditioning control is given in U.S. Pat. No. 5,975,994 issued in November 1999 to Sandhu, et al. It suggests monitoring the distribution over the pad surface of such its characteristics as thickness of wear debris and other polishing waster and pad contour, and then selectively conditioning only the non-uniform areas of the pad surface. The selective conditioning is achieved, for instance, by changing the down-force and speed of the conditioner over various pad areas. This method controls pad flatness and, to a lesser degree, surface cleanliness, but it does not address other crucial characteristics of pad wear and surface condition.
A method and apparatus for measuring the thickness loss of a polishing pad is described in the U.S. Pat. No. 6,354,910 issued in March 2002 to R. Adebanjo et al.
The method uses rigid planar members placed on the surfaces of both the conditioned and non-conditioned sections of the polishing pad. Measurements are made utilizing measurement tools, which overhang the depressed conditioned section, and measure actually the height difference between the upper surfaces of the planar members. This method provides information about pad thickness in just several pre-determined locations rather than over the whole working area, and also does not monitor the pad surface condition.
A single sensor for pad control is suggested in the U.S. Pat. No. 6,040,244 issued in March 2000 to Arai, et al. This sensor scans the pad before and after polishing, simultaneously measuring pad thickness and contour, as well as its surface roughness. This method, however, does not provide for in-situ, in-process pad control.
By nature, the removal of material during both polishing and conditioning is caused by surface interactions or friction. Both polishing and conditioning processes to a great extent depend on such factors as friction characteristics of the materials, surface conditions of the wafer, conditioner and the polishing pad, the rate of wear of the polishing pad, and the rate of removal of the wafer material. Thus, for control and optimization of polishing processes, it is necessary to experimentally monitor the wear and friction characteristics of polishing pads in real time and actual polishing conditions.
An attempt of a friction-based CMP process control based on measuring the running motor current is described in U.S. Pat. No. 5,948,700 issued in September 1999 to Zheng, et al. This technique is not applicable for accurate measurements of forces and torques and for polishing process control. Since no load current flows through the electric motor when no load is applied thereto, it is difficult to accurately detect the level of friction developed on the platen. Furthermore, the current flowing through the motor greatly depends on the voltage of corresponding power supply and on speed of rotation. Therefore even small variations in the power supply voltage and changes in the rotation speed cause significant changes in the current. In addition, since in the aforementioned CMP system both the platen and the wafer holding device are connected to respective motors through corresponding transmissions, accuracy of friction measurements based on the motor current may be affected by losses and slippage in the transmissions.
A method and apparatus for controlling a polishing process described in U.S. Pat. No. 5,738,562 issued on Apr. 14, 1998 to Doan, et al. are based on measurement of variations that occur in translational (lateral) motions of the polishing platen, related to the variations in friction of different film materials. These method and apparatus are based on indirect measurement technique, result in very approximate evaluation of the friction variations, cannot accurately measure the friction coefficients and thus, are not suitable for practical control of the CMP process. Also, they do not measure pad wear.
Another polishing control method is disclosed in U.S. Pat. No. 5,948,205, issued in September 1999 to Kodera, et al. It comprises steps of measuring friction between the layer being polished and a turntable carrying a polishing slurry during polishing, determining the polishing rate from the measured friction, determining the extent of polishing by integrating the polishing rate over time, and terminating the polishing operation when the measured polishing extent coincides with a predetermined value. This method assumes that the friction and the rate of polishing are in direct relationship, which is not typically a fact. Also, it monitors neither a pad conditioning process nor pad wear.
An apparatus and a method for conditioning monitoring in CMP are disclosed in WO Patent No. 01/15865 A1, issued in March 2001 to Moore. A CMP machine contains a conditioner attached to a support and a force sensor connected to the conditioner support for measuring a friction force in the interface between the conditioner and a polishing pad. The apparatus allows for monitoring the conditioning process. However, the frictional force can be a function of the surface characteristics of the pad and conditioner, as well as a function of the normal compression force and the relative velocity between the two surfaces. Also, this method does not monitor pad wear. Additionally, this apparatus is complicated and expensive, as the force sensor has to be integrated in the conditioner assembly.
A method of polishing pad control is described in U.S. Pat. Nos. 5,650,619 and 5,825,028, issued to Hudson in July 1997 and October 1998, correspondingly. It is based on covering the pad with a special indicating compound and detecting defective pads by changes in the compound. Though this method may allow for separation of good and bad polishing pads, it is not applicable for in-situ conditioning and pad wear control.
Another method of pad conditioning control is suggested in U.S. Pat. No. 6,234,868, issued in May 2001 to Easter, et al. It consists of the load control via a magnetic mechanism. Though this method may allow for pad conditioning, it does not have any feedback on pad surface conditions and wear, which limits its accuracy and usefulness.
Two U.S. Patents issued to Robinson, et al., U.S. Pat. No. 6,090,475 of July 2000 and No. 6,136,043 of October 2000, suggest a polishing pad, which changes its color when worn out. This may allow for detection of the time of plate replacement, though it makes the pad more expensive. This technique, however, allows to monitor neither dynamics of pad wear nor any crucial pad surface conditions.
A method of pad conditioning control is described in U.S. Pat. No. 5,664,987, issued in September 1997 to Renteln. It includes measurements of the removal rate on just polished wafers and comparison of the measured data with a predetermined level. When the removal rate drops below this level, the pad is conditioned. Though providing a rather common way of conditioning control, this method lacks immediate, real-time feedback for adjusting the conditioning process, and as a result, one or several wafers may be under-polished or over-polished before the pad is optimally conditioned. Also, it does not indicate when the pad is worn out and has to be replaced.
An apparatus for polishing pad contour monitoring, which measures the contour of a polishing surface of the pad, is disclosed in U.S. Pat. No. 5,618,447 issued in April 1997 to Sandhu. The monitor consists of a displacement sensor, which is attached to a supporting member assembled rigidly over the pad. The sensor has a vertical guide and a sliding pin on its working tip, continuously sliding on the rotating pad and thus measuring the pad contour. This apparatus may be sufficient to characterize pad contour and wear, but it does not address any other important conditions of a pad surface, directly affecting polishing results.
A similar apparatus for in-process pad control is described in U.S. Pat. No 5,834,645 issued in November 1998 to Bartels, et al. A contact probe interrogates the pad and generates a control signal when the measured displacement of its contact stylus exceeds a pre-selected threshold, thus indicating the presence of an extraneous material. The probe can have an angular or translational motion and may include plurality of optical fibers. This apparatus may be sufficient to characterize pad contamination, but it does not address pad wear and other important conditions of a pad surface, directly affecting polishing results.
A non-contact pad control is suggested in U.S. Pat. No. 6,045,434 issued in April 2000 to Fisher, et al. The non-contact interferometry measurements of pad thickness and wear are conducted either during or between polishing cycles, with the measured data used as a feedback signal for both polishing and pad conditioning control. Either ultrasound or electromagnetic radiation transmitters and receivers are aligned to cover the full radial length of the pad. This method may be sufficient, though expensive, to characterize pad wear, but it does not monitor any other important conditions of a pad surface, directly affecting polishing results.
A similar pad control is described in U.S. Pat. No. 6,194,231, issued in February 2001 to Ho-Cheng, et al. A radial pad profile is monitored with a linearly scanning device, and when the profile change exceeds a predetermined limit, the pad has to be changed. Again, this method may be sufficient, though expensive, to characterize pad wear, but it does not monitor any other important conditions of a pad surface, directly affecting polishing results.
A pad conditioning control suggested in U.S. Pat. No. 6,093,080 issued in July 2000 to Inaba, et ale is based on measurements of either a frictional force between the pad and wafer or a corresponding torque current. The conditioning process is adjusted so as to make the frictional force in polishing constant; these adjustments can be done in conditioning load, time or number of conditioner revolutions. This method may allow for process control, but it has several drawbacks. Firstly, it does not monitor pad wear. Secondly, it may be too complicated and expensive to measure the polishing friction force between pad and wafer. Thirdly, as will be shown later in the text, frictional force is not a so stable process characteristic as the coefficient of friction. Lastly, monitoring torque current may not be accurate due to the effects of friction in bearings, transmission and other components of the mechanical system of the polishing apparatus.
The U.S. Pat. No. 6,257,953 issued to N. Gitis et al. in July 2001, as well as U.S. U.S. Pat. No. 6,494,765 issued to the same authors in December 2002 provides an apparatus and a method for monitoring the polishing and conditioning processes using coefficient of friction sensors and acoustic emission sensors, and performing direct monitoring of both pad-wafer and pad-conditioner interfaces. However, these methods and apparatuses have the sensors and signal electronics built into the polishing machine, which is complicated and expensive. Also, they do not monitor pad wear.
The above described methods and apparatuses do not combine measurements of pad wear with frictional or acoustic parameters, and as a result, cannot be effectively used in semiconductor production for making process decisions by separating different process problems. Indeed, if measuring Pad friction or acoustics alone, when the friction or acoustic parameter deviates out of process specification, the operator has insufficient information to distinguish whether it is due to a worn pad which has to be replaced, or to a worn conditioner which has to be either cleaned or replaced, or to a defective incoming wafer. Similarly, if measuring pad wear alone, the operator cannot wait for replacing the pad when wear has reached a critical depth, as it is common when polishing uniformity and removal rate deteriorate much earlier than when the critical Pad wear death has been reached. Therefore, a demand for more accurate and comprehensive control of pad conditioning by monitoring pad wear and its surface condition still exists, as the known methods and apparatus do not provide full control and comprehensive measurements of pad surface conditions inherent in a polishing process
It is an object of the present invention to provide effective, accurate, universal, and reliable method and apparatus for monitoring polishing plate condition and wear and thus control of a plate conditioning process in polishing. Another object is to provide an apparatus for monitoring polishing plate condition as a separate unit, which does not require changes in a polishing machine and is capable to monitor condition of a polishing plate currently installed on the polishing machine. Another object is to provide method and apparatus for monitoring polishing plate conditions and wear and thus to control a polishing process, including a CMP process, by differentiating process problems. It is another object to provide a method and apparatus for polishing plate monitoring on the basis of combined direct wear, frictional and acoustical measurements. It is another object to provide an inexpensive, easy to use, aforementioned apparatus of the type that can be built into any polishing, grinding, or lapping machine without need in structural modification of such a machine. Yet another object is to provide an apparatus and method for a CMP process with controlled conditioning of the polishing plate surface.
The invention provides an apparatus and a method for monitoring a polishing plate condition, capable of simultaneous measurements of compression and friction forces, acoustic emission signal developed in contact between special probes and a polishing plate, as well as polishing plate wear, which is a result of both polishing and conditioning procedures. The apparatus is not a part of a machine the polishing plate is installed on, though can be attached to it in order to monitor condition of a currently installed polishing plate. The apparatus comprises a measuring unit, being in contact with the polishing plate, and a data processing unit. The measuring unit contains a set of sensors, which can include wear sensors, friction force sensors, compression force sensors, and acoustic emission sensors. The sensors measure compression force, friction force, acoustic emission level, and wear of a polishing plate. The data processing unit acquires the data signals from the measuring unit and computes process parameters, such as friction coefficient, wear rate, etc. The apparatus makes it possible to monitor the condition of the polishing plate surface, affecting the polishing, and to detect the moment when the plate needs to be replaced.
Additionally, the invention provides the aforementioned apparatus and method, wherein the sensors are installed on both a working zone of a polishing plate surface and its non-working zone. This method allows for monitoring the condition of the working zone of the polishing plate in comparison with its non-working zone.